1. Field of the Invention
This invention relates to a process for producing a rare earth element-iron-boron magnet that may generate a strong static magnetic field in vacant spaces of a magnetic circuit mounted in a motor, which is used for example in audio visual systems, office automation systems, etc. More particularly, it relates to a process for producing a rare earth element-iron-boron magnet comprising applying pressure to a billet of a rapid solidification powder of a rare earth element-iron-boron alloy while allowing a current to pass through the billet to conduct a plastic deformation. Said rapid solidification powder is a mixture of crystalline structures and amorphous structures.
2. Description of the Prior Art
Said motor requires a magnet with high residual induction and thermal stability. For that reason, an Sm-Co type sintered magnet has been used in the past. However, in recent years a rare earth element-iron-boron sintered magnet with high residual induction which is produced according to a powder metallurgy method has been utilized as described in Japanese Patent Publication No. Laid Open 61-111514, which may urge the development of a compact motor with high output powder.
On the other hand, it is well known that a magnetic material is subjected to a plastic deformation for providing its texture with magnetic anisotropy, thereby obtaining excellent magnetic properties in a certain direction. For example, a texture of grain oriented silicon steel plate which is formed by a roll magnetic anisotropy method is well know. Also, there are other magnets known in the art, for example, a Mn-Al-C, and rare earth element-iron-boron rapid solidification magnets which are made by plastic deformation techniques of magnetic materials such as an extruding procedure, die-upsetting procedure, and the like. These magnetic anisotropy techniques are based on a principle that when the magnetic material is subjected to plastic deformation, there is a certain relationship between the direction of stress or strain and the axis of easy magnetization of the crystals of the materials.
A rare earth element-iron-boron rapid solidification magnet is produced by (1) melting a raw material mixture of Nd, Fe, B, and other added elements as needed, (2) rapid solidification the melted mixture by a melt-spinning procedure to form a rapid solidification flake powder of fine crystal having an R.sub.2 TM.sub.14 B phase, wherein R is neodymium(Nd) and/or praseodymium(Pr), TM is iron(Fe) and/or cobalt(Co), and B is boron, as a major phase with a grain size of about 20-500 nm, (3) applying pressure to the rapid solidification powder to form a billet at high temperatures, and then (4) subjecting the billet to plastic deformation (Japanese Laid-Open Patent Publication 60-100402). For example, the raw material mixture is melted by an arc discharge, and then rapidly solidified by a single roll method under Ar atmosphere to form a rapid solidification flake powder with a thickness generally of 20-30 .mu.m which is a mixture of crystalline structures and amorphous structures. The rapid solidification powder is roughly ground to 32 mesh or less. Then, a pressure of about 3 tonf/cm.sup.2 is applied to the ground rapid solidification powder in a hard metal mold. The pressed rapid solidification powder is then subjected to high-frequency heating at a temperature of 600.degree. to 800.degree. C. while applying a pressure of 0.5-2 tonf/cm.sup.2 to form a billet having full density. Then, the billet is placed in a mold cavity of the hard metal mold of which the surface is treated with a graphite lining, and then subjected to plastic deformation under a pressure of 0.5-2.5 tonf/cm.sup.2 during a high-frequency heating to form a rare earth element-iron-boron.
Although these conventional plastic deformation techniques disclose the production of magnets with magnetic anisotropy, they do not disclose any particular method for forming a specific shape, or maintaining magnetic properties.
According to this plastic deformation technique, the billet of the rapid solidification powder is held for a long period of time such as several hundred seconds at temperatures higher than its crystallization temperature, or heated to an elevated temperature of 750.degree. C. or more, the fine crystals of the rapid solidification powder become larger, which may cause the reduction of intrinsic coercivity.
The thermal stability of magnetic properties such as intrinsic coercivity of magnet depends generally on the value of the intrinsic coercivity, and its temperature coefficent. When the intrinsic coercivity of magnet is larger, or its temperature coefficient is smaller, the thermal stability of the magnet will be improved. For example, the intrinsic coercivity of a sintered magnet has been increased to 20 kOe by the addition of dysprosium(Dy) or other elements in order to maintain its thermal stability. However, when the intrinsic coercivity of the magnet is increased, the residual induction for an applied magnetic field is reduced, which makes it difficult to generate a strong static magnetic field in vacant spaces of a magnetic circuit. Therefore, it is desirable that the thermal stability of a magnet be improved by reducing the temperature coefficient of intrinsic coercivity of the magnet. For instance, a rare earth element-iron-boron sintered magnet or rapid solidification magnet with a maximum energy product of 30 MGOe or more has the temperature coefficient of intrinsic coercivity of -0.6%/.degree.C. The above value of about -0.6%/.degree.C. is not sufficiently low as a temperature coefficient of the intrinsic coercivity, therefore, because of a great demagnetization by heat, the stability against the heat from a magnetic circuit cannot be sufficiently secured. For that reason, reducing the temperature coefficient to -0.5%/.degree.C. or less is preferable. However, it is difficult to reduce the temperature coefficient of intrinsic coercivity of the rapid solidification magnet to -0.5%/.degree.C. or less by conventional plastic deformation techniques.
Also, when a sintered magnet is produced by these plastic deformation techniques, its magnetic properties may be damaged in the plastic deformation process for providing it with magnetic anisotropy. In the case of the sintered magnet, an alloy powder ground to the size of particle which can become a single magnetic domain is formed in the magnetic field before sintering. When a small and thin magnet is formed in the magnetic field, however, the orientation of the particle in the magnetic field tends to be disturbed due to the compressed pressure because the direction of the magnetic field is identical to that of the compression. For example, it is difficult for a thin magnet or about 1 mm thickness to obtain a residual induction of 11 kg or more because of the disturbed orientation of the particle.